In the 1950's, the power generation industry widely used ferrous NiMoV, similar to American Society of Testing Materials ("ASTM") 293 class 5 material, to make Low pressure ("LP") and High Pressure ("HP") Steam turbine components, such as rotors. Steam turbine components, including rotors, are not currently made of ferrous NiMoV alloy because this material exhibits inferior elevated temperature properties as compared to other ferrous low alloy steels such as Chromium Molybdenum Vanadium ("CrMoV").
Nevertheless, turbine components comprised of ferrous NiMoV low alloy steel are still in service in the field of power generation, some in operation for over 30 years. Due to prolonged use, NiMoV alloy turbine components, in particular, the surfaces of HP turbine rotors, may become creep damaged or corroded. Rather than replacing an entire turbine rotor, for example, the damaged sections of the rotors are generally repaired. A welding process is commonly used to perform repairs of rotors. Welding processes are generally economical and have been effective at extending the operating life of rotors.
For example, single rotor blade groove fasteners, known as steeples commonly require repair. Over time, especially in HP environments, the steeples are subject to creep, corrosion, pitting, etc. . . . It is common to machine down creep damaged or corroded steeples. Then, weld metal is deposited onto the machined surface of the rotor. Finally, the build-up of weld metal is machined to form new or repaired steeples. This process does not work well, however, for repairing steeples on ferrous NiMoV low alloy steel HP turbine component. Known welding processes have been found to weaken the heat affected ferrous NiMoV low alloy steel adjacent to the weld fusion zone making the components unusable in high pressure and elevated temperature environments.
In detail, filler or weld metal is deposited onto a creep damaged or corroded area/surface of the rotor by reusing the filler or weld metal with the surface. Gas tungsten arc welding ("GTAW"), plasma-arc welding, electron beam welding, laser-beam welding or gas metal arc welding may be used to deposit the weld metal. See, for example U.S. Pat. Nos. 4,893,388, 4,897,519, 4,903,888, and 4,940,390 which are assigned to the assignee of this application (these patents teach repairing and replacing creep damaged steeples and other damaged areas on the surface of ferrous CrMoV low alloy steel steam turbine components such as rotors and are hereby incorporated by reference for their teachings on methods of repair and welding techniques).
The first layer of weld metal deposited on the surface of a turbine component fuses with the surface of the component. The area or line between the fusion zone and base metal of a turbine component is known as the fusion line. GTAW is commonly used to deposit the first layer of weld metal. The GTAW process uses arc to fuse the weld metal to the turbine component. The arc also elevates the temperature of the base metal in the area adjacent to the fusion line.
Consequently, the microstructure and mechanical property of the base metal in this area are significantly changed. This area is known as the Heat Affected Zone ("HAZ"). A portion of the metal in this area is reaustenitised and dramatically cooled due to the welding thermal cycle causing the metal to be hardened creating a hardened sub-zone of the HAZ. The metal closest to the point of fusion is generally subject to the greatest level of hardening. On the other hand, a portion of the metal in this area is tempered during the welding thermal cycle causing the metal to be softened. This softened sub-zone of the HAZ is located farther away from the point of fusion line than the hardened sub-zone.
Using normal process of depositing layers of weld metal, it has been found that the level of hardness of ferrous NiMoV low alloy steel around the fusion line in the HAZ can be as high as 50 on a Rockwell "C" scale ("Rc"), however, the level of hardness of the softened sub-zone can be as low as 240 on a Kneep scale ("HK") (18 Rc). The normal level of hardness of ferrous NiMoV low alloy steel of a turbine component is approximately 25 Rc. Ferrous NiMoV low alloy steel with a level of hardness of 50 Rc is highly suspectable to cracking. Ferrous NiMoV low alloy steel with a level of hardness of 18 Rc has very low elevated temperature strength and very low creep resistance.
In order to relieve welding thermal induced stress and the level of hardness of the HAZ, the weld area is normally heat treated after the deposition of the layers of weld metal (postweld). In particular, ferrous NiMoV turbine components are normally heat treated at a temperature of 1200.degree. F. (649.degree. C.) for ten hours. After this postweld heat treatment, the level of hardness of the ferrous NiMoV low alloy steel around the fusion line in the HAZ is reduced, in some cases as low 36 Rc. It has been found that the level of hardness of the low alloy steel around the fusion line can be further reduced by using higher temperatures during the postweld treatment. Higher temperatures, however, may produce carbine coarsening or overtempering of low alloy steel in the HAZ that was not hardened or reaustenitised, i.e., softened sub-zone, during the welding process.
Overtempering further aggravates the softened sub-zone in HAZ of the low alloy steel, i.e., it further reduces the level of hardness of the softened sub-zone below the normal level of ferrous NiMoV low alloy steel; i.e., 16 Rc. This lowers the creep strength of the low alloy steel making the turbine component unusable in high pressure/temperature applications. As a consequence, it is not common to repair ferrous NiMoV low alloy steel turbine components used in HP environments using welding processes. Thus, a need exists for a repair process for worn or damaged ferrous NiMoV low alloy steel steam turbine components, such as rotors used in HP environments. In particular, a process that does not substantially affect the level of hardness of the ferrous NiMoV low alloy steel turbine component.